Labile blood pressur constitutes an apparent risk factor for hypertension whose determinants are unknown. The proposed research has as its objective an evaluation of genetic and psychological contributions to lability of blood pressure. Genetic factors may contribute to variability in blood pressure by modulating the reactivity of the sympathetic nervous system, and personality factors, perhaps gene-influenced, may underlie vulnerability to environmental stress. Monozygotic and like-sex dizygotic twins, aged 18-24, will be taught to monitor their blood pressures at home. Six pressures will be taken daily over a one-month period. Each subject will visit the laboratory where his pressures will be monitored repeatedly throughout a standardized protocol of relaxation and arousal. Variability of the casual blood pressure data will be analyzed for evidence of genetic variance, and the profiles of blood pressure during standardized stress will be subjected to repeated measures analysis of variance to assess genetic contributions to lability of blood pressure. Each subject will complete two psychometric questionnaires of coronary-prone behavior. Scores from these tests will be analyzed for genetic variance and will be correlated with measures of blood pressure lability. The research will test the hypothesis that vascular reactivity to standardized stress can predict the level of blood pressure lability experienced in the natural environment. The potential contribution of this research is new information on the interaction of genetic predisposition and personality structure in determining blood pressure lability and early hypertension.